The present invention relates to the recovery from stall of a gas turbine engine, and, more particularly, to a method and apparatus for promoting a surge condition in such a stalled engine, during which surge condition the stall-inducing agent can be eliminated and the engine can thus resume normal operation.
A typical gas turbine has an inlet portion for drawing in ambient air. Such air is thereupon compressed by an axial flow compressor and passed to further stages of the gas turbine engine. While the air is being compressed in the compressor, it normally has an unseparated flow pattern throughout the length of the compressor. Various stall-inducing agents, however, such as an abrupt increase in fuel flow to the combustor, can result in material disruption of such an unseparated flow pattern whereby a separated flow pattern, characterized by a high degree of turbulence, is created. The functioning of such a gas turbine engine subject to a stall-inducing agent is then said to be stalled. In such a condition, the output power of the engine falls off considerably. The post stall behavior of the engine can be characterized by either a surge condition or a non-surge condition. A "surge" condition is a post stall response in which an engine continually alternates between normal operation and stalled operation. A non-surge condition, on the other hand, is a condition in which the engine tends to operate in a rotating stall mode.
Difficulties can arise when a gas turbine engine becomes stalled and operates solely in a non-surge condition. A first difficulty is that of having to wait for the stalled gas turbine engine to regain normal power output while an operator of the engine causes the engine to go through shutdown and restart modes. The duration of this wait can be dangerously long where a stalled engine constitutes the propulsion means for an aircraft.
A further difficulty of a stalled gas turbine engine operating in a non-surge condition is that it may become overheated and cause damage to the turbine stage thereof. Such overheating can occur because the compressor of a stalled gas turbine engine draws in much less air than normal through the inlet portion of the engine. Meanwhile, the combustor stage of the engine can continue to add a large amount of heat to the now diminished flow of air that passes therethrough. Consequently, a large and destructive amount of heat can be imposed on the turbine stage of a stalled gas turbine engine.
It has been recognized in the prior art that the presence of a surge condition following stall initiation in a gas turbine engine is desirable because the responsible stall-inducing agent can be eliminated during the surge condition, thereby enabling the engine to readily assume normal operation. See for example, E. M. Greitzer, "Surge and Rotating Stall in Axial Flow Compressors; Theoretical Compression System Model", Transactions of the ASME, J. Engrg. for Power, Vol. 98, No. 2, April 1976, pp. 190-198, at 190. However, the prior art as indicated, for example, in the foregoing article teaches that the promotion of a surge condition in a stalled engine falls within the ambit of the engine designer. While engine design certainly can help to ensure that a surge condition will exist in a stalled engine, it would be desirable to provide a method and apparatus for promoting a surge condition in a stalled engine which is effective in any type of engine design. Such a method and apparatus could be incorporated in existing gas turbine engines by retrofitting the engines.